The present invention relates to a sewing machine having a machine arm, a rest for the material being sewn and a device, which comprises a cutting knife and a cutting block, for producing buttonholes, especially eyelet buttonholes, in the material being sewn, the device being acted upon by an actuating device that can be moved from an upper position into a lower position, the cutting knife being arranged in a stationary manner underneath a stitch plate and the cutting block being able to be lowered onto the cutting knife.
Such a sewing machine is marketed by the Applicant under the description Durkopp Adler Type 558, for example. Comparable sewing machines are disclosed by German Patent 291 045, German Patent 817 993 and German Offenlegungsschrift 35 33 022.
Such machines are basically provided for the purpose of using them to sew and to cut buttonholes of different lengths and shapes. In addition, the machines are able to operate in a pre-cutting mode and a post-cutting mode. In the pre-cutting mode, the buttonhole is cut before the buttonhole stitching is produced, and in the post-cutting mode the buttonhole stitching is produced (sewn) first and the buttonhole is then cut.
The cutting block and the cutting knife are always matched in the cutting unit, since over the duration of the operation the cutting knife cuts into the cutting block and thus leaves behind a depression which always has to be filled by the cutting knife in order to produce a clean and complete cut.
At the time of converting from the pre-cutting mode into the post-cutting mode, it is necessary not only for the knife to be offset in its position in the longitudinal direction, but also for the cutting block associated with this position to be inserted. Consequently, two cutting blocks are provided for each knife. Although they are of identical construction and are arranged at identical positions, they also have knife impressions that are positioned differently because of the different positioning of the knife. Conversion and adjustment consequently require a corresponding expenditure of time and effort.
In order to be able to sew in different modes, the sewing machine disclosed by DE 291 197 has a plurality of cutting knives which are provided in the baseplate, have different lengths and can be moved into engagement with a single cutting block. The reliable functioning of a machine configured in this way is only ensured when the cutting knives of different lengths are used regularly and interchangeably, so that none of the cutting knives--in particular not the shortest cutting knife--can dig more deeply into the cutting block than the others, with the result that each cutting knife finds its position in the cutting block again during the cutting. However, such a precondition is purely theoretical. If a shorter knife digs more deeply into the cutting block than the longer knife or knives, it is not possible to make any proper incisions using the longer cutting knives.